[Proj] How to use +towgs84

Aaron Friesen aaron at cartopac.com
Thu Mar 5 15:26:08 EST 2009

> -----Original Message-----
> In choosing WGS84 as a "pivot datum" PROJ.4 is replicating a relatively
> common practice with other GIS software though this approach does have
> some pitfalls.
> It is possible to use +towgs84 to apply datum shifts that aren't really
> to or from WGS84 (likewise with +nadgrids) but this must be done with
> some care and caution since the library conceptually considers it is 
> using WGS84 as the pivot datum.  So trick the library carefully if you
> wish/need to do this.  If someone wants to carefully consider how to do 
> this and document it, it would be a nice addition to the FAQ list in Trac.

Just to clarify, however, if I am taking for example UTM NAD83 to latlong
NAD83 without a datum shift, I do not need +towgs84 specified.  Correct?


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